1. Field of the Invention
The present invention relates to a high-frequency heating apparatus for heating such objects as foods and fluids, and, in particular, relates to a high-frequency heating apparatus using a semiconductor electric power converter for generating high-frequency power in an electric power supply section thereof.
2. Description of the Related Art
High-frequency heating apparatus such as a home or domestic microwave oven often uses a power circuit of a construction such as shown in FIG. 1. In FIG. 1, when an operation switch 1 is turned on, a commercial power supply 2 is connected to a high-voltage transformer 3. The output of the secondary winding of the high-voltage transformer 3 is rectified by a capacitor 5 and a diode 6 and is supplied to a magnetron 7. A heater winding 8 of the high-voltage transformer 3 is connected to the cathode of the magnetron 7 to heat the cathode. As a result, the magnetron 7 is caused to oscillate and produces a high-frequency electromagnetic wave (radio wave), thereby making it possible to effect induction heating.
FIG. 2A is a diagram showing a change with the passage of time of the radio wave output P.sub.0 of the magnetron 7 after turning on the switch 1 at a time point t=0. When the switch 1 is turned on at time point t=0, the magnetron 7 is supplied with cathode heating power and high-voltage power at the same time. At time point t.sub.1 about one or two seconds later, the cathode temperature sufficiently increases and the radio wave output P.sub.0 rises, and thereafter the radio wave output is kept substantially constant as shown in FIG. 2A. The radio wave output may, of course, decrease to some degree with the lapse of time due to such factors as the temperature characteristic of the magnetron 7 and the high-voltage transformer 3. Nevertheless, the radio wave output P.sub.0 (for example, 500 W) predetermined as a rated output of the particular apparatus is basically maintained.
FIG. 2B is a diagram showing the increase in temperature of the internal parts of a high-frequency heating apparatus after operation of the heating apparatus has been started. The temperature T.sub.M of the magnetron 7 and the temperature T.sub.a of the ambient air of the high-voltage transformer 3 increase in such a manner as shown in this diagram.
FIG. 3 is a sectional view of a high-frequency heating apparatus. A housing 9 has an oven 10, a magnetron 7, a high-voltage transformer 3, etc. arranged therein in the manner shown in FIG. 3, and they are forcibly cooled by a cooling fan 11. The efficiency of the magnetron 7 is about 60% and that of the high-voltage transformer 3 is about 90%, so that in the case of an apparatus with an actual radio wave output rating of 500 W, the magnetron 7 develops a loss of about 300 W and the high-voltage transformer 7 about 100 W. As a result, the temperature of these parts gradually increases during the operation as shown in FIG. 2B. The rate of this temperature increase is comparatively high up to a time point of t.sub.2 (say, 15 minutes) determined by the thermal time constant of each part, and thereafter the temperature of the whole apparatus reaches a maximum temperature level at time point t.sub.3 (say, in 60 to 120 minutes) where the temperature becomes saturated.
In this way, a high-frequency heating apparatus is composed of parts such as the magnetron 7 and the high-voltage transformer 3 which have comparatively low conversion efficiency and hence generate much heat loss, thus causing a comparatively high temperature increase during its operation and taking a long time before it reaches a stable temperature.
In order to guarantee the rated output P.sub.0 of a heating apparatus, it is necessary to have the heating apparatus constructed by using insulating material and component parts capable of maintaining sufficient safety against such a heat loss. For this reason, the cooling conditions and the specifications of the parts of the apparatus are designed to meet the guarantee requirements. Specifically, the component materials, parts specifications and cooling structure are determined by sufficiently taking into consideration the temperature rise that occurs at the time point t.sub.3 in FIG. 2B.
The cooling conditions and the parts specifications for the rated output of 500 W are greatly different from those for the rated output of 600 W. The generation of a loss by the magnetron 7, for example, differs between both rated outputs, so that, in the case of the rated output of 600 W, the magnetron 7 must have a bulky cooling construction, which results in an increase in the size and cost thereof, and the high-voltage transformer 3 is also obliged to have a large size and increased cost.
Thus, in a conventional high-frequency heating apparatus, as described above, the specifications of respective component parts are determined in such a manner as to guarantee safety and reliability of the apparatus under a temperature condition where the temperature rise caused by the heat loss has saturated.
However, high-frequency heating apparatus uses a special method of heating called induction heating, and the heating time thereof is comparatively short. It is often used with a very short heating time of less than five minutes when it is used for the purpose of reheating which is often the case with general home applications. That is, as shown in FIG. 2B, many operations come to end within the time t.sub.0 an or so, and a temperature corresponding to the time to t.sub.3 occurs rarely. Thus, most conventional high-frequency heating apparatuses that guarantee the temperature rise thereof to be reached at t.sub.3, which is not required at all under many operating conditions, are operated within the time t.sub.0. Thus, it can be said that such apparatuses have excessively high quality. Nevertheless, this substantially excessive quality has hitherto been considered as unavoidable in view of rare cases where the apparatus is operated under an operating condition that reaches the time point t.sub.3.